2,3,7,8-Tetrachloro-dibenzo-p-dioxin (TCDD) and related xenobiotics produce their biological and toxic effects by binding to a gene regulatory protein, the Ah receptor (AhR), and inappropriately modulating gene expression. Long-term goals are to understand the function of the AhR, its regulation, and the mechanisms by which it modulates gene expression. Achievement of these goals will significantly enhance our ability to evaluate both the health risks from environmental levels of these xenobiotics and the possible relationship of AhR-regulated processes to other disease states. It is hypothesized that phosphorylation pathways directly and indirectly regulate AhR activity and its function as a transcription factor. It is further hypothesized that these pathways play a major role in tissue-, species-, and temporal-specific responses to these chemicals. This program will focus specifically on examining the mechanisms whereby phosphorylation pathways regulate ligand binding to the AhR, heterodimerization with another protein (Arnt), binding to specific DNA sequences and transcriptional activity. Changes in the phosphorylation state of the AhR-Arnt complex determines its DNA-binding activity. Studies will determine the mechanism for this effect and whether the action is on the AhR and/or Arnt. Using a number of molecular approaches and site-directed mutagenesis, studies will determine the specific phosphoamino acids and AhR and Arnt, and begin to examine the regulatory functions of these phosphorylations. Data indicates that phosphatases affect transcriptional activity of the AhR. Additional studies will determine if this effect is directly on the AhR-Arnt complex or on accessory transcriptional factors. Finally, additional studies will define nuclear factors that influence AhR-Arnt transcriptional activity and determine how altered phosphorylation of these affect this activity.